1. Field of the Invention
The present invention relates to an optical pickup device and/or an optical disc device which is capable of optically recording information on and reproducing the information from an information recording medium such as an optical disc by using a laser source.
2. Description of the Background Art
In recent years, among DVDs and next-generation high-density optical discs, an optical disc having two or more recording layers has been proposed in order to increase a recording capacity per optical disc.
FIG. 1 is a block diagram showing a basic configuration of an optical pickup device. FIG. 8 is a block diagram showing a configuration of a conventional optical system used in the optical pickup device, the optical system corresponding to reference character 13 shown in FIG. 1.
The optical pickup device shown in FIG. 1 records information on and reproduces the information from an optical disc 206 having two information recording layers, i.e., a first information recording layer 206a and a second information recording layer 206b. In the optical system 13 shown in FIG. 8, three beams, i.e., a zeroth-order diffracted ray (main beam), and two first-order diffracted rays (sub beams) are generated by a diffraction grating 202 which is arranged in an optical path of a light beam outputted from the laser source 201. The two first-order diffracted rays are located on both sides of the zeroth-order diffracted ray. The generated three beams pass through a beam splitter 203, a collimator lens 204, and an objective lens 205, and then form three light spots on the first information recording layer 206a of the optical disc 206. In this case, suppose that the beams are in focus on the first information recording layer 206a. A reflected main beam and reflected sub beams, which are reflected by the first information recording layer 206a of the optical disc 206, pass through the objective lens 205, the beam splitter 203, and the detection lens 207, and then enters a photodetector 208.
FIG. 3 shows, in detail, a configuration of the photodetector 208 shown in FIG. 1. As shown in FIG. 3, the photodetector 208 includes a main beam detecting section 301 which receives the reflected main beam 30 reflected from the optical disc 206, and sub beam detecting sections 302 and 303 which respectively receive the reflected sub beams 31 and 32 reflected from the optical disc 206. The photodetector 208 performs photoelectric conversion on the each of the light beams received at each of the detecting section, and outputs an electrical signal corresponding to intensity of each of the light beams.
The main beam detecting section 301 and the sub beam detecting sections 302 and 303 are respectively divided into four detecting elements. The divided detecting elements 301a, 301b, 301c, 301d, 302e, 302f, 302g, 302h, 303i, 303j, 303k, and 303l output signals A, B, C, D, E, F, G, H, I, J, K, and L, respectively. By using the signals A to L, a focusing error signal FE is detected by a known astigmatic method, and a tracking error signal TE is detected by a known push-pull method. More specifically, in accordance with the signals A to D outputted from the main beam detecting section 301, a main signal calculation circuit 304 generates a main push-pull (MPP) signal and a main focusing error (MFE) signal. In accordance with the signals E to L outputted from the sub beam detecting sections 302 and 303, a sub signal calculation circuit 305 generates a sub push-pull (SPP) signal and a sub focusing error (SFE) signal. The control signal calculation circuit 306 generates a tracking error (TE) signal, a focusing error (FE) signal, and an information (RF) signal.
The aforementioned calculations performed in the main signal calculation circuit 304, the sub signal calculation circuit 305, and the control signal calculation circuit 306 are executed according to the following equations 1 to 7.MPP=(A+D)−(B+C)  (1)MFE=(A+C)−(B+D)  (2)SPP={(E+G)−(F+H)}+{(I+K)−(J+L)}  (3)SFE={(E+H)−(F+G)}+{(I+L)−(J+K)}  (4)TE=MPP−α×SPP  (5)FE=MPP+β×SPP  (6)RF=A+B+C+D  (7)Wherein, α and β represent constant numbers.
In accordance with the TE signal and the FE signal, which are generated by the control signal calculation circuit 306 in the photodetector 208, a control signal processing circuit 101 shown in FIG. 1 outputs a control signal to an objective lens drive circuit 102 so as to drive an objective lens drive unit 103, and causes the objective lens 205 to move in a thickness direction and a radial direction of the optical disc 206.
One of techniques relating to the above-described optical pickup device is disclosed in Japanese Laid-Open Patent Publication No. 2001-307351.